A study from Harvard Medical School shows that a high rise in blood sugar after meals can limit increases in your ability to take in and use oxygen during exercise (Nature Metabolism, July 20, 2020). This means that if your diet causes high blood sugar levels when you eat, you lose one of the major health benefits of exercise (JAMA Intern Med, 173, 1834–1836 (2013). People who are considered “non-diabetic” because their fasting blood sugar levels are normal (<100 mg/dL), but who have high blood sugar levels after eating, are at increased risk for diabetes and heart attacks no matter how much they exercise.

The Study
• One group of mice was fed a diet high in sugar and saturated fat, which made them develop high blood sugar levels and gain weight.
• Another group of mice was fed a diet that restricted saturated fat and sugar so they did not gain weight, but they were given a drug to block insulin so they developed high blood sugar levels.
• Both groups ran 500 kilometers (311 miles), a significant amount of exercise. In spite of this vigorous exercise program, neither group increased their maximal ability to take in and use oxygen. They also did not have the growth of new blood vessels in muscles that would be expected from this exercise program.
The authors then found exactly the same results in humans: Those with high blood sugar levels after meals had the lowest increase in ability to use oxygen when they exercised.

My Recommendations
Type II diabetes is largely a lifestyle disease caused by an inflammatory diet, lack of exercise and excess weight. Seventy percent of North Americans will become diabetic, and 30 percent don’t know they are diabetic because their fasting blood sugar levels are normal (<100 mg/dL). In my opinion, you should be told that you are diabetic if your blood sugar level rises over 145 mg/dL one hour after meals, even if your fasting blood sugar is normal (Diabetes Technology & Therapeutics, Sept 2017;19(9)).

These new studies remind us that you cannot expect exercise alone to control high blood sugar levels. You can help to control blood sugar levels with a diet that restricts foods with added sugar, all sugared drinks, foods made from ground up grains and other processed foods (BMJ Open, 2016;6:e009892). I also recommend eating a wide variety of vegetables, fruits, whole (unground) grains, beans, nuts and other seeds.